The present invention relates to a cell for pumping a multiphase effluent and to a pump comprising such a cell or several of such cells mounted in series. A multiphase effluent is understood to be an effluent consisting of a mixture of at least two phases selected from (a) a liquid phase consisting at least of one liquid, (b) a gas phase consisting at least of a free gas, and (c) a solid phase consisting of particles of at least one solid suspended in (a) and/or (b).
Multiphase pumping is a technology used in many industrial sectors, such as petroleum and gas production (pumping of a petroleum two-phase effluent consisting of a mixture of oil and of gas), the chemical industries, the nuclear industry (pumping of a mixture of water and of steam), and spacecrafts.
The base architecture of industrial pumps used for multiphase effluent pumping includes an impeller (or hydraulic wheel) followed by a stator (or static diffuser). The function of the impeller is to transmit kinetic energy to the mixture to be pumped, the static diffuser then performing transfer of the mixture under pressure, in particular to the impeller of the cell located immediately downstream in the case of a pump comprising several pumping cells.
Theoretical studies and tests have shown that there is a relation between the liquid single-phase pumping head (HL) and the multiphase effluent pumping head (HPh):
xe2x80x83HPh=Exc3x97HL
where E is the multiphase efficiency, which is essentially a function of the void fraction xcex1 and of the pressure at the inlet,                     (            ⁢      α        =                  Q        G                              Q          G                +                  Q          L                      ,
QG and QL being the pumping speeds of the gas G and of the liquid L respectively).
Application of conventional pumps (centrifugal, axial-flow or semiaxial-flow pumps) to pumping of a mixture of water and steam has been studied in the nuclear field for one-stage pumps (impeller and stator) in order to be able to face up to an exceptional accident in a reactor. The tests that have been carried out on that occasion show that the two-phase pumping efficiency E greatly decreases as soon as the void fraction a exceeds 0.15-0.20 and, consequently, the multiphase pumping head (HPh) loses 80% of its liquid single-phase value( HL), which leads to a multiphase efficiency E=0.2. The main cause is due to the separation of the phases: the liquid particles are centrifuged in the impeller, forming a thin liquid film on the external wall. This liquid film moves along the external wall of the impeller and of the stator, which leads to a fall in the kinetic energy of the multiphase effluent and to a degradation of the multiphase pumping head (HPh).
On the basis of this experiment, the petroleum and gas industry has studied a helical-axial flow impeller wherein the centrifugation effect is limited and, consequently, part of the liquid phase is kept dispersed in the gas, thus leading to a higher multiphase efficiency E=0.5 to 0.8, for an inlet pressure above 10 bars. For low gas ratios, combination of a helical-axial flow staged pump followed by a centrifugal pump has been proposed for pumping at the bottom of oil wells, in FR-A-2,748,533.
However, this result is relative because of the fact that the liquid pumping head (HL) of the helical-axial flow impeller is low in relation to that of semiaxial-flow pumps, so that, globally, the multiphase pumping head (HPh) obtained by the two systems is comparable.
Furthermore, at low pressures (2-3 bars), the multiphase efficiency (E) of existing industrial impellers (semiaxial-flow pumps as well as helical-axial flow pumps) becomes very low (E being then about 0.1), which is disadvantageous for practical use.
The aim of the present invention is to propose a pump comprising at least one multiphase pumping cell capable of providing an interesting liquid pumping head (HL), (which is currently the case for semiaxial-flow pumps, but not for helical-axial flow pumps) while having a good multiphase efficiency (E) (which is currently the case for helical-axial flow pumps, but not for semiaxial-flow pumps).
The present inventor therefore has discovered that, contrary to the idea which naturally occurs, according to which the phases should not be separated, and which is applied in the case of helical-axial flow pumps, at least partial separation of the phases in the impeller could be accepted and that one could take advantage of the transmission of the high kinetic energy of the liquid film to the effluent to be pumped, provided that dynamic means, and not static means, ensuring the mechanisms of homogenization of the phases and of their energy levels, then of pressure recovery and finally gas compression are provided. Most of these means are not implemented in the existing systems once they have transmitted the kinetic energy to the (more or less separated) phases, because the stators used in existing pumps are not suited to fulfil these functions. In particular, these conventional stators do not ensure the process of energy exchange between the phases and are limited to transfer of the flows to the outlet in the configuration of more or less separated phases, which leads to a great degradation of the multiphase efficiency (E).
The object of the present invention is thus first a cell for pumping a multiphase effluent, characterized in that it comprises two rotary parts, the first part consisting of a hydraulic wheel designed for transmitting kinetic energy to each phase of the multiphase effluent entering the cell, and the second part, following the first, consisting of an energy converting device designed for homogenizing the phases, transferring kinetic energy between the phases, entraining the lightest phase, converting kinetic energy into pressure and compressing the homogeneous effluent before it leaves said cell, all of said rotary parts being mounted on a common shaft axially arranged inside a fixed housing comprising an inlet and an outlet for the multiphase effluent.
The two components of the pumping cell according to the present invention are thus rotary, unlike existing industrial systems, the second component fulfilling, in a new and original way, in combination, several rebalancing functions in relation to the effects due to a partial separation of the phases, also allowed, in a new and original way, by the first component.
The hydraulic wheel forming the first rotary part of a pumping cell according to the present invention generally consists of a boss mounted on the axial shaft and carrying blades exhibiting a hydrodynamic profile to allow transmission of kinetic energy to the multiphase effluent, the blades forming, between the housing and the boss, channels whose length is sufficiently great to provide the kinetic energy level required for carrying the multiphase effluent.
The energy converting device forming the second rotary part of a pumping cell according to the invention consists, according to a particularly interesting embodiment, of at least one continuous or discontinuous helical wheel carried by a boss mounted on the axial shaft and which rotates in an energy homogenization and transfer chamber delimited by the housing and having a section orthonormal to the axis substantially larger than the sum of the sections orthonormal to the axis of the channels of the hydraulic wheel, the extended length of said helical wheel or of said helical wheels being sufficiantly great for the kinetic energy homogenization and transfer efficiency required for pressure recovery.
The energy converting device must first homogenize the phases. In the case of a gas-liquid mixture, this means that the liquid particles must entrain the gas, transmitting kinetic energy thereto. Mixing must therefore be long enough, a function that is fulfilled by the helical wheel(s), a dynamic mixer, capable of homogenizing the phases. Once the mixture homogenized, conversion of kinetic energy into pressure is obtained by means of a significant speed decrease due to the increase in the section of the chamber. Finally, the chamber-helical wheel(s) system is such that it simultaneously provides compression of the homogeneous effluent, mainly of its gas phase, before it leaves the cell, and this effect can be intensified if the angle of the helical wheel(s) is varied by increasing it in the direction of the cell outlet.
All these functions essential for energy recovery of the pressure in the multiphase effluent are specific to the present invention. The conventional stator of existing industrial systems does not provide the exchange process between the phases and it is limited to transfer of the flows to the outlet in the configuration of more or less separated phases, which leads to a degradation of multiphase efficiency E.
After thus describing the preferred main characteristics of the two rotary parts forming the pumping cell according to the invention, non-limitative particular embodiments of the geometry of these two parts are described hereafter:
The ratio of the section orthonormal to the axis of the energy homogenization and transfer chamber of the energy converting device to the sum of the sections orthonormal to the axis of the channels of the hydraulic wheel is notably 3 to 10.
The or each continuous or discontinuous helical wheel of the energy converting device extends over an angle of at least 270xc2x0 and advantageously makes a complete turn.
Two or three continuous or discontinuous helical wheels can also be provided for the energy converting device; they are then advantageously and evenly axially shifted and exhibit an angular displacement in relation to one another of 180xc2x0 and 120xc2x0 respectively.
The angle of inclination of a or of each helical wheel of the energy converting device in relation to a plane perpendicular to the shaft in the direction of the pumping cell outlet is advantageously of the order of 10xc2x0, and it can increase at the outlet where it can be 20xc2x0.
The hydraulic wheel can have a constant or variable diameter, the ratio of the outside diameter (Ds) of said wheel at the outlet to its outside diameter at the inlet (De) being notably 1 to 3. Unlike well-known helical-axial flow impellers, the outside diameter of the hydraulic wheel of the pumping cell according to the present invention can increase in the direction of the outlet in order to intensify the kinetic energy transfer to the phases.
Furthermore, in relation to well-known semiaxial-flow impellers, it can be noted that the length of the channels of the hydraulic wheel of the cell according to the present invention is sufficient for energy phenomena to be stabilized, which means that partial separation of the phases is accepted. Under such conditions, in the case of a gas-liquid effluent, the liquid particles whose kinetic energy is highly concentrate in the vicinity of the external wall and, at the outlet of the hydraulic wheel, inside a channel, there is gas, followed by a gas-liquid mixture and by a liquid layer outside.
The channels are advantageously identical, their number can for example range between 4 and 10. Their length is notably,       k    xc3x97                  De        +        Ds            2        ,
k ranging between 0.5 and 1.5.
A static or dynamic flow diffuser device is preferably provided to ensure good distribution and continuity of the flow at the outlet of the hydraulic wheel of a pumping cell over the total section of the energy homogenization and transfer chamber of the associated energy converting device; such a device can advantageously consist of a grate with hydrodynamic profiles carried by the housing and mounted in said chamber, between the inside of the housing and the helical wheel(s).
The present invention also relates to a pump comprising a multiphase pumping cell as defined above, or several of these cells mounted in series, the shaft carrying the rotary parts being common to all the cells. The number of these pumping cells is selected to provide the multiphase pumping head required for the application considered.
It can also be noted that the pump according to the invention can readily fit already existing mechanical pumping structures, the rotary parts, respectively the specific impeller and the rotary energy converting device, of a or of each cell according to the invention respectively replacing the impeller and the static diffuser of an existing cell, the existing structure of the housing elements, of the shaft and of the bearings being maintained.